Optical fiber Bragg gratings are important elements for selectively transmitting or reflecting specific wavelengths of light within an optical fiber. A fiber Bragg grating comprises a length of optical fiber containing a refractive index profile that varies periodically along the length of the fiber. Refractive index variations with a single period, .LAMBDA., selectively reflect light with a wavelength of .lambda.=2.LAMBDA.. Other wavelengths are transmitted essentially unimpeded. Alternatively, A can be chosen to vary along the length of the fiber in order to reflect a broad range of wavelength, e.g. chirped gratings. Such broadband gratings can for example be used for dispersion compensation to provide a wavelength dependent time delay to a propagating signal with a finite bandwidth. Another class of fiber gratings comprises the long-period-gratings in which the periodic spacing is at least 10 times larger than the transmitted wavelength, i.e. .LAMBDA.&gt;10.lambda.. These gratings provide wavelength dependent losses by coupling optical power between co-propagating guided and non-guided modes. Long-period gratings remove selected wavelengths from the guided mode into the non-guided mode and consequently spectrally shape the transmitted beam (U.S. Pat. No. 5,764,829) while causing little back-reflection in the fiber. Fiber gratings in general have numerous applications in the areas of optical sensing, signal processing, spectral filtering, and optical communications.
Simple periodic fiber gratings are known in the art, and many different methods have been described for impressing refractive index gratings in the core of photosensitive (e.g. germanium-doped) optical fibers (U.S. Pat. No. 4,474,427) including holographic techniques (U.S. Pat. No. 4,725,110), phase mask techniques (U.S. Pat. No. 5,367,588), and internally-reflecting prisms (U.S. Pat. No. 5,377,288). In addition, methods have been described for producing chirped fiber Bragg gratings (U.S. Pat. No. 5,718,738), fiber gratings possessing a continuous sinc function envelope on a periodic index of refraction modulation (U.S. Pat. No. 5,668,901), and methods for impressing an aperiodic grating on an optical fiber (U.S. Pat. No. 5,388,173).
Many present optical communication systems utilize diffraction gratings to enhance their performance. Fiber gratings are for example advantageous in wavelength division multiplexing (WDM) systems in which fiber Bragg gratings can provide high reflectivity and high wavelength selectivity with the aim of increasing the transmission capacity of optical fibers. Co-pending patent applications Ser. Nos. 08/403,376 and 60/070,684, and 08/897,814 which are referenced above describe a technology which relies on complex diffraction gratings for increasing the capacity of optical systems by utilizing a different type of multiplexing which can be termed optical code division multiple access (hereinafter OCDMA). OCDMA systems encode different communication channels with different temporal (time) codes as contrasted to the coding in WDM systems wherein different channels use different wave lengths of light.
Co-pending patent application Ser. No. 09/100,592 describes segmented surface diffraction gratings which consist of multiple subgratings, each subgrating having a specific amplitude, spatial phase, and spatial period. Such surface gratings can deflect optical pulses from a specific input direction to a specific output direction while simultaneously multiplying the Fourier spectrum of the input pulse by a predetermined filtering function. The output signals are a cross-correlation between the input waveform and the grating encoded temporal waveform. The gratings described in the referenced co-pending applications have a complex profile. They can accept input beams and generate spectrally filtered output beams propagating in one or more output directions. The filtering function of the device is programmed by choice of grating profile. By suitable programming, multiple transfer functions may be realized, each having its own specific input and output direction.
The present invention relates to fiber gratings with complex refractive index grating profiles, specifically segmented fiber gratings capable of providing programmed spectral filtering with high efficiency. The previous art does not encompass the segmented fiber gratings pursuant to the present invention. Another aspect of the present invention relates to methods for fabricating segmented fiber gratings. In another aspect of the present invention, the complex fiber gratings are used in an OCDMA optical communication system.